An expanded polytetrafluoroethylene (hereinafter abbreviated as “PTFE”) product formed using PTFE has a microstructure comprising fibrils and nodes interconnected with each other by the fibrils. Expanded PTFE products are porous for such microstructures, and therefore, are also called “porous PTFE products”
Properties of an expanded PTFE product, such as pore size and porosity, can be controlled primarily by relying upon stretching conditions. Among such expanded PTFE products, expanded PTFE tubes which may hereinafter be called “porous PTFE tubes” are provided with properties derived from their porous nature, such as flexibility, fluid permeability, fine particulate capturing capacity, low dielectric constant and low dielectric dissipation factor, in addition to properties such as heat resistance and chemical resistance and surface properties such as low friction coefficient, water repellency and non-tackiness, all of which are possessed by the material PTFE itself. For these unique properties, the utility of expanded PTFE tubes is not limited only to the general industrial field but is also spreading to the medical field and the like.
Taking a porous PTFE tube, for example, it is rich in flexibility and its material PTFE itself is excellent in antithrombic property, and moreover, its porous structure based on a microfibrous structure formed as a result of the stretching and comprising a number of fibrils and a number of nodes interconnected with each other by the fibrils is excellent in biocompatibility. Expanded PTFE tubes, therefore, have found wide-spread utility as substitute blood vessels for maintaining circulation, for example, to replace lesion parts of blood vessels in living bodies, especially to bypass such lesion parts.
A porous PTFE tube is generally produced by mixing a liquid lubricant with unsintered powder of PTFE, forming the resulting mixture into a tubular shape by ram extrusion, drying off the liquid lubricant, and then expanding the tubular extrusion product by stretching in the direction of its axis. Subsequent to the expanding, the expanded extrusion product is heated to a temperature of the melting point of PTFE or higher while holding it to avoid shrinkage, so that the expanded structure is sintered and fixed. When the stretching temperature is sufficiently high, the sintering and fixing is effected concurrently with the expanding step.
Despite such various excellent properties as mentioned above, porous PTFE tubes have a strong molecular orientation in the direction of extrusion and tend to tear in the direction of their longitudinal axes. Porous PTFE tubes are, therefore, accompanied by a problem in that, when blood vessels in living bodies are shunted using porous PTFE tubes as artificial blood vessels, the tubes may tear in the direction of their longitudinal axes by suture needles or sutures to induce hematoma formation or false aneurysm due to blood leakage. This problem becomes particularly pronounced when upon production of porous PTFE tubes, the stretch ratio is increased to make the porosity higher, the pore size is made greater, or the wall thickness is reduced.
As a method for providing an expanded PTFE tube with higher axial tear strength, it may be contemplated to perform the stretching of an extrusion product in biaxial directions, that is, in the longitudinal direction and in the radial direction. With this method alone, however, it is still impossible to achieve any substantial improvement in the axial tear strength.
A process was therefore reported in JP-B-43-20384 and JP-B-7-15022. According to that process, extrusion is conducted while providing an extrusion product with an orientation at an angle with respect to the direction of a longitudinal axis by producing a helical flow in the extrusion product with a helical groove formed on a die or mandrel of a ram extruder.
In the above-described process, however, the extrusion product is stretched in the direction of its longitudinal axis in a subsequent step so that the intersecting angle between the direction of the orientation and the direction of the longitudinal axis becomes too small to expect any substantial improvement in the axial tear strength. Especially when stretching at a high stretch ratio of 4 times or more in the longitudinal direction is needed to produce a porous PTFE tube having a high porosity of 70% or more, the direction of orientation becomes closer to the direction of the longitudinal axis so that practically no effect is expected in increasing the axial tear strength.
With a view to obtaining a porous PTFE tube having high axial tear strength, it was also proposed to reduce the porosity or to reinforce a porous PTFE tube by helically winding an expanded PTFE tape on an outer surface of the porous PTFE tube (JP-B-52-9074). Nowadays, one of two methods is adopted, one being to lower the porosity of a porous PTFE tube, and the other being to helically wind an expanded PTFE tape or filament on the outer surface of a porous PTFE tube such that the porous PTFE tube is reinforced.
However, the method, which relies upon a reduction in the porosity of a porous PTFE tube or a reinforcement by a tape or filament wound on the outer surface of a porous PTFE tube, involves a problem that some inherent characteristics of a porous PTFE tube, such as flexibility and tissue cells invasion, are impaired although the method is effective in increasing the axial tear strength.